Storage.read alway get the fixed data?

I want to use Storage to cache datas.

After the some times, I always get the fix length data which I write very early.
I want to know that do Storage can overwirte the old datas when I user write("",""). Otherwise, why I always read the very first datas? And the data length is always 38.

console.log("iiiiiiiiiiiiiiiiiiiiiiiiiiiiii_1", JSON.stringify(cac))

  let cac = st.read("http");
  console.log("iiiiiiiiiiiiiiiiiiiiiiiiiiiiii_0", JSON.stringify(line))
  console.log("iiiiiiiiiiiiiiiiiiiiiiiiiiiiii_1", JSON.stringify(cac))
  if (cac) {
    line = line.substr(line.indexOf(tar_flag) + tar_flag.length);
    console.log("iiiiiiiiiiiiiiiiiiiiiiiiiiiiii_2", JSON.stringify(line))
  }
  cac += line;
  console.log("iiiiiiiiiiiiiiiiiiiiiiiiiiiiii_3", JSON.stringify(cac))
  st.write("http", cac);

st.getFree()
=248900